Cycles image texture node is writen in python or c?
Implemented in C, see:

The whole tree:
tree/HEAD:/source/blender/nodes/shader/nodes

The Mapping node:
HEAD:/source/blender/nodes/shader/nodes/node_shader_mapping.c
The image texture node:
HEAD:/source/blender/nodes/shader/nodes/node_shader_tex_image.c

Editing / changing is possible, but then you embark on a journey of custom compiles whenever you want to update, maintaining your own branch might not be an option until you have a certain amount of C / compile / revision control experience.